Technical Field
This invention relates to improving the long term stability, or retention of physical properties, of polyacetal compositions containing a non-meltable polymer stabilizer. Specifically, the overall long term stability of polyacetal compositions containing a non-meltable polymer stabilizer is found to be significantly improved when there is incorporated into the composition a minor amount of at least one meltable co-stabilizer, along with at least one primary antioxidant.
Polyacetal compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Polyacetal compositions containing a non-meltable polymer stabilizer possess excellent stability during processing. Compositions based on polyacetals of relatively high number average molecular weight, e.g., 10,000 to 100,000, which contain a non-meltable polymer stabilizer, are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping and thermoforming. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance. However, the physical properties of the polyacetal composition containing the non-meltable polymer stabilizer may decrease with time, even when there is present an antioxidant. It is therefore an object of the present invention to improve upon the long term stability (i.e., improve upon the long term retention of physical properties) of a polyacetal composition containing at least one non-meltable polymer stabilizer. Such an improvement is achieved through the incorporation into the polyacetal composition of a minor amount of at least one meltable co-stabilizer, said co-stabilizer being a thermal stabilizer for polyacetal, and at least one primary antioxidant.
The compositions of the present invention are useful wherever enhanced stability, both during and after processing, are desired.